Weather is ultimately caused by temperature differences due to variations in solar energy input. These temperature differences cause pressure variations in the atmosphere. The wind blows to try and correct these imbalances in pressure. Regions of low and high pressure moving through an area cause changes in weather as different air masses pass by.
Why do I care? Understanding high and low pressure systems helps determine how the weather is likely to change over time.
From the faintest breeze that gently shakes the leaves on the trees to the powerful gusts that tear buildings apart, wind is a part of our everyday lives. However, not many people take the time to think about what makes wind blow and why it’s important.
The rotation of the earth does have an effect on the direction of the wind, but it does not create it. Wind is primarily driven by differences in air pressure. These variations in air pressure are due to temperature differences caused by variations in solar energy received at the surface of the earth. The sunlight absorbed by the ground depends on the latitude of the location, the slope and the underlying surface. For example, soil heats up more rapidly than water because of the greater heat capacity of water than soil.
The air pressure on Earth is not the same everywhere. There are areas of high and low pressure. The wind blows from higher pressure to lower pressure in an attempt to make the pressure in the two areas equal. Image 1 in the diagram shows an area of low pressure. This means that the low pressure center is surrounded by higher pressure on all sides. This would mean the wind blows directly in toward the center of the low. Initially, this is correct. However, since the earth is rotating, the Coriolis force comes into play. In the Northern Hemisphere, this turns the wind toward the right. This means that wind blowing toward the south would be turned so that it would end up blowing toward the west, wind that started out blowing toward the west would end up blowing toward the north and so on, as shown in image 2 in the diagram. The Coriolis force is what causes low pressure systems to rotate counterclockwise. Friction slows the wind and causes it to turn slightly toward lower pressure. This causes the wind to blow in toward the center of a low pressure system, as shown in image 3. Because all the wind flows towards the center of the low pressure area, we say it is converging. In the center, since the air cannot go down, it rises, which leads to clouds and precipitation.
The same rules apply to a high pressure system. The high pressure center is surrounded by lower pressure on all sides, so the wind blows directly out from the center at first, as shown in image 4. Then, the Coriolis force turns the wind toward its right, causing clockwise rotation around a high pressure system, as shown in image 5. Friction makes the wind turn slightly toward lower pressure, which in this case is out away from the center of the high pressure area. This makes the wind blow slightly outward from the center of a high, as shown in image 6. Diverging air in the center of the high pressure leads to clear skies and no rain.
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Activities to accompany the information above:
Activity: Renewable Energy: Wind (pdf version of original activity.)
Description: This activity focuses on the relationship between barometric pressure, wind speed and wind direction. Students will use these parameters to analyze surface maps of the United States and form a relationship with the patterns that will show up on the map.